Conventional snowmobile drive trains incorporate a variable ratio belt drive system (commonly referred to as Continuously Variable Transmission or CVT) having a driving pulley that is directly coupled to the engine crankshaft and a driven pulley coupled to an output shaft. The driving pulley acts as a clutch and includes a centrifugally actuated adjusting mechanism through which the drive ratio of the belt drive is varied progressively as a function of the engine speed and the output torque of the driven pulley. Typically, the driven pulley is coupled to a transverse jackshaft which in turn drives the input member of a chain and sprocket reduction drive. The output of reduction drive is coupled to one end of the axle on which are located the drive track drive sprocket wheels. Conventional jackshafts are typically solid steel shafts having splined ends onto which the driven pulley and the reduction drive are connected.
The splined ends or key way of the jackshaft are inserted into adapters that connect the jackshaft to the driven pulley and to the input member of the chain and sprocket reduction drive. A first adapter has an inner surface machined grooved to mesh with the first splined end of the jackshaft and an outer surface machined to engage the driven pulley. A second adapter has an inner surface similarly machined grooved to mesh with the second splined end of the jackshaft and an outer surface machined to engage the input member of the chain and sprocket reduction drive. FIG. 6 illustrates a prior art driven pulley mounted onto such a first adapter. As can be seen, the prior art driven pulley 200 is mounted onto an adapter shaft 202 which includes a series of internal splines designed to receive the splined ends of the conventional jackshaft. The prior art driven pulley 200 includes an axially stationary sheave 203, an axially moveable sheave 204 and a fixed outer portion 205. An internal biasing mechanism 206 is mounted between the fixed outer portion 205 and moveable sheave 204 to maintain the groove 207 at its minimum width. In operation, the moveable sheave 204 adjusts the width of the groove 207, and therefore the effective radius of the driven pulley 200, as a function of the torque applied to the driven pulley 200. The connection of the prior art pulley to a conventional jackshaft is therefore cumbersome and requires numerous parts, adding weight and cost to a snowmobile. The heretofore required use of an adapter to connect a conventional jackshaft to the driven pulley and to the reduction drive increases the number of parts included in the drive train as well as the number of steps required for the assembly thereof. This increases the production cost of the snowmobile as well as increases the number of part to maintain in inventory. Neither of which is desirable.
Furthermore, conventional jackshafts are made of solid steel shafts to resist the high torque generated between the snowmobile engine and the drive track. These solid shafts perform generally well for this application. Conventional solid jackshafts have been bored through in order to reduce their weight but the wall thickness remains very thick for the purpose of machining their splined ends. Thus they remain heavy. As is well known in the art of snowmobile design, the lighter the snowmobile, the better.
Thus, there is a need for a snowmobile jackshaft that alleviates some of the drawbacks of conventional jackshaft design.